Tunable Filter for RF Circuits

ABSTRACT

A tunable filter is described where the frequency response as well as bandwidth and transmission loss characteristics can be dynamically altered, providing improved performance for transceiver front-end tuning applications. The rate of roll-off of the frequency response can be adjusted to improve performance when used in duplexer applications. The tunable filter topology is applicable for both transmit and receive circuits. A method is described where the filter characteristics are adjusted to account for and compensate for the frequency response of the antenna used in a communication system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority with U.S. ProvisionalApplication Ser. No. 61/922,645, filed Dec. 31, 2013; the contents ofwhich are hereby incorporated by reference.

FIELD OF INVENTION

The present invention relates generally to the field of wirelesscommunication; and more particularly, to tunable filters as applied toRF front-end configurations in communication systems and dynamicadjustment of bandwidth characteristics of RF circuits.

BACKGROUND OF THE INVENTION

As the need for higher data rates increases, communication systems arebeing designed to cover wider frequency bandwidths as well as a largernumber of frequency bands. The introduction of 4G protocols such as LongTerm Evolution (LTE) are a main driver in the increase in additionalfrequency bands being used for cellular communication systems. Thecomplexity of the RF front-end topology of communication systems isincreasing due to the need for backward compatibility with 2G and 3Gprotocols as 4G LTE capability is introduced. In addition, Advanced LTEas a protocol is configured to accommodate carrier aggregation, wheremultiple channels can be transmitted or received on simultaneously toincrease instantaneous bandwidth. This aggregation of channels can coverup to five channels spread across multiple frequency bands. Carrieraggregation that utilizes multiple frequency bands points to a need fordynamic tuning of various components of the RF front-end including thefilters to provide the flexibility needed to access various frequencypairings. All of these trends point toward a growing need for moreflexibility in the RF front-end of mobile communication systems toaddress the combining of multiple frequency bands and modes.

Dynamic tuning of components that comprise the RF front-end ofcommunication systems is picking up adoption in the commercialcommunications industry, and proper implementation of dynamic tuningmethods can bring improvements to communication system performance asthe number of frequency bands that can be accessed grows and theinstantaneous bandwidths required increases.

The requirement to design a mobile device such as a cell phone thatcovers multiple frequency bands and multiple modes of operation forcesthe system designers to develop a front-end transceiver circuit thatcombines several power amplifiers (PA) on the transmit side and severallow noise amplifiers (LNA) on the receive chain. Typically, each PA andLNA requires a filter to reduce spurious emissions and harmonics.Without the filters in the circuit, the PA will amplify unwantedfrequency components which can fall within the frequency band of receivefunctions. On the receive side, the absence of filtering will cause theLNAs to amplify unwanted frequency components which can result in anincrease in the noise floor in the receive chain. This increase in noisefloor can result in reduced Signal to Noise Ratio (SNR) which in turncan result in an increase in Bit Error Rate (BER), with the end resultbeing a decrease in data rate for data transmission.

The filters used in current commercial communication systems have afixed frequency response. The start and stop frequencies which establishthe instantaneous bandwidth as well as the center frequency are fixed.The conventional technique for implementing filters in a communicationsystem is to determine the frequency bandwidth required from the filteralong with the slope of the skirts (the roll-off in performance as afunction of frequency) for a specific function and location within thecircuit topology. A filter that meets the frequency responserequirements is designed, manufactured, and implemented in the circuit.Very good filtering can be achieved using SAW (Surface Acoustic Wave),BAW (Bulk Acoustic Wave), and FBAR (Film, Bulk Acoustic Resonator)filter types in a fixed filter implementation. The main drawback is theinability to dynamically alter or tune a filter response once it isimplemented in a circuit. A tunable filter would provide the capabilityof dynamically adjusting the bandwidth of a transmit or receive circuitto track changes in bandwidth for LTE waveforms. LTE provides for arange of bandwidths for a data stream based upon the amount of dataneeded to transmit or receive and priority of the data stream in thecellular network. LTE bandwidths can vary from 1.5 MHz to 20 MHz. Withcarrier aggregation being implemented in LTE-Advanced (LTE-A), there isnow a potential of up to five channels being aggregated to increaseinstantaneous bandwidth to 100 MHz. A tunable filter would provide thecapability of matching the bandwidth of the communication systemfront-end to the instantaneous bandwidth of the LTE waveform.

SUMMARY OF THE INVENTION

A method of designing a tunable filter that can dynamically adjust thecenter frequency and bandwidth characteristics of the filter isdescribed. Novel circuit topologies are disclosed that provide thecapability to adjust the center frequency of the filter withoutaffecting the bandwidth characteristics, adjust the bandwidthcharacteristics without affecting the center frequency, or adjust boththe center frequency and bandwidth characteristics simultaneously.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A-B) illustrate a tunable filter formed by two operationalamplifiers, where variable impedances are coupled to the non-invertinginput of the first operational amplifier.

FIGS. 2(A-B) illustrate a typical frequency response for a band passfilter, with the pass band, reject band, and roll-off region defined.

FIGS. 3(A-B) illustrate an example of a tunable filter circuit inaccordance with an embodiment.

FIGS. 4(A-B) illustrate an example of a tunable filter circuit inaccordance with another embodiment.

FIGS. 5(A-B) illustrate an example of a tunable filter circuit inaccordance with another embodiment.

FIGS. 6(A-B) illustrate an example of a tunable filter circuit inaccordance with another embodiment.

FIG. 7 illustrates a pair of tunable filters configured in an RF systemfor use in filter signals in the transmit and receive paths.

FIG. 8 illustrates a total of eight tunable filters configured with fourtunable filters connected to four power amplifiers and four tunablefilters connected to four low noise amplifiers.

FIG. 9 illustrates a filter module.

FIGS. 10(A-B) illustrate a method of implementing a look-up table inmemory in a processor to control a PA, tunable filter, tunableattenuator, and switch.

FIGS. 11(A-B) illustrate a method of implementing an algorithm in memoryin a processor to control a PA, tunable filter, tunable attenuator,power detector and switch.

FIGS. 12(A-B) illustrate a method of implementing an algorithm in memoryin a processor to control an LNA and tunable filter.

FIGS. 13(A-B) illustrate the front-end of a communication system wheretunable filters are implemented in both the transmit and receive chains.

FIGS. 14(A-B) illustrate the front-end of a communication system wheretunable filters are implemented in both the transmit and receive chains.

FIG. 15 illustrates a tunable filter and antenna circuit simulation.

FIG. 16 illustrates a tunable filter topology, showing an operationalamplifier and components to provide the tuning function.

FIG. 17 illustrates a detailed circuit topology of the operationalamplifier.

FIG. 18 illustrates the transmission loss characteristics of the tunablefilter and antenna circuit shown in FIG. 15.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment is realized in an RF circuit topology wherein twooperational amplifiers are used in a configuration with a tunablecapacitor connected to the non-inverting input of the first operationalamplifier. The tunable capacitor is used to alter the center frequencyof the band-pass filter formed by the circuit. By connecting the tunablecapacitor in shunt between the non-inverting input of the firstoperational amplifier and ground, a change in capacitance of the tunablecapacitor will translate into an inverse change in frequency of thefilter response of the circuit. The tunable capacitor can be implementedusing one of several different techniques or types such as a MEMSdevice, switched capacitor assembly fabricated in CMOS, Silicon onInsulator (SOI), Silicon on Sapphire (SOS), or Gallium Arsenide (GaAs),a Barium Strontium Titanate (BST) capacitor, or varactor diode.

In another embodiment a variable resistor circuit is connected to theoutput of the second operational amplifier, with this variable resistorcircuit connected in turn to the non-inverting input of the firstoperational amplifier. A change in resistance of the variable resistorcircuit will translate into an inverse change in frequency of the filterresponse of the circuit. One implementation of a variable resistorcircuit can be a pair of multi-port switches with fixed resistorsbetween port pairings, such that the switches can be activated in unisonto allow switching from one port to the next.

In another embodiment both the tunable capacitor connected to thenon-inverting input to the first operational amplifier and the variableresistor circuit connected between the output of the second operationalamplifier and the non-inverting input of the first operational amplifierare tuned in conjunction to adjust the frequency response of the filtercircuit. The product of the resistance and capacitance will directlytranslate to an inverse change in frequency of the filter response ofthe circuit.

In another embodiment a variable resistor circuit is connected in seriesconfiguration to the non-inverting input of the first operationalamplifier prior to the junction of the capacitor that is connected inshunt configuration between the non-inverting input and ground. Thevariable resistor circuit can be used to change the bandwidth of thefilter response of the circuit by adjusting the resistance. Thebandwidth is inversely proportional to the resistance of the variableresistor circuit.

In another embodiment the variable resistor circuit used to change thebandwidth is adjusted in conjunction with the variable capacitor andvariable resistor circuit used to change the center frequency of theband pass response of the filter circuit. Adjusting these threecomponents or circuits simultaneously or sequentially provides thecapability of dynamically adjusting the center frequency and bandwidthcharacteristics of the filter circuit. With a properly designed tunablefilter the center frequency can be adjusted while the bandwidth is keptconstant; the center frequency can also be adjusted while the bandwidthis varied. The variable resistor circuit can also be adjusted tomaintain a near constant transmission loss of the tunable filter as afunction of frequency of operation.

In yet another embodiment a tunable filter can be designed andimplemented in a system to take into account the frequency response ofthe antenna used to transmit and receive RF signals. Optimizing thetunable filter to work with a specific antenna can result in improvedout of band performance when the antenna is designed to have a sharperfrequency roll-off. By implementing a tunable antenna and tunable filtercombination, a look-up table can be implemented to tune both the filterand antenna per frequency channel.

FIG. 1A illustrates a tunable filter formed by two operationalamplifiers, including first operational amplifier 11 and secondoperational amplifier 12, where variable impedances are coupled to thenon-inverting input of the first operational amplifier. The variableimpedances provide the ability to adjust the center frequency of thefilter response. The output of the second operational amplifier 12 iscoupled into the non-inverting input of the first operational amplifier11 through one of the variable impedances 13 for frequency control. Athird variable impedance 14 for bandwidth control is located at theinput 15 to the dynamic frequency and bandwidth control circuit 16 andis used to dynamically adjust the bandwidth of the filter response.

FIG. 1B shows a plot illustrating a plurality of tuned frequencyresponses (F1; F2; . . . ; Fn) compared to the conventional use of afixed filter frequency response. The instant tunable filter allows for aplurality of tuned frequency responses.

FIG. 2A illustrates a typical frequency response for a band pass filter,with the pass band, reject band, and roll-off region defined. In FIG.2B, the filter characteristics of a tunable filter are shown, where thefrequency response is adjusted or tuned to cover different frequencyranges.

FIG. 3A illustrates an example of a tunable filter circuit where avariable capacitor 31 is coupled to the non-inverting input of firstoperational amplifier 11. The tunable circuit further includes a secondoperation amplifier 12, resistors (R1; R2; R3; R4; and R5), staticcapacitor (c), input port (In) and output port (Out) as shown. Thetunable capacitor can be adjusted to vary the center frequency of thetunable filter. The frequency response of the tunable filter for threetuning states, C₁, C₂, and C_(n) are shown in FIG. 3B.

FIG. 4A illustrates an example of a tunable filter circuit where avariable capacitor 31 is coupled to the non-inverting input of firstoperational amplifier 11 for providing a variable reactance (C1, C2, . .. , Cn). A variable impedance frequency control (VIFC) network is alsocoupled to the non-inverting input of first operational amplifier 11.The resistance of the variable impedance frequency control network canbe adjusted to vary the center frequency of the filter. The tunablefilter circuit further includes a second operation amplifier 12,resistors (R1; R2; R3; R4; and R5), static capacitor (C), input port(In) and output port (Out) as shown. The frequency response of thetunable filter for three tuning states, R₆, R₇, and R_(n) are shown inFIG. 4B.

FIG. 5A illustrates an example of a tunable filter circuit where avariable capacitor 31 is coupled to the non-inverting input of firstoperational amplifier 11 for providing a variable reactance (C1, C2, . .. , Cn). A variable impedance frequency control (VIFC) network is alsocoupled to the non-inverting input of first operational amplifier 11. Asecond variable impedance network, variable impedance bandwidth control(VIBC) network, is coupled to the input port of the filter circuit. Theresistance of the first variable impedance network can be adjusted inconjunction with the tunable capacitor to vary the center frequency ofthe filter. The tunable filter circuit further includes a secondoperation amplifier 12, resistors (R1; R2; R3; R4; and R5), staticcapacitor (C), input port (In) and output port (Out) as shown. Thefrequency response of the tunable filter for three tuning states, R₁,R₂, and R_(n) are shown in FIG. 5B.

FIG. 6A illustrates an example of a tunable filter circuit where avariable capacitor 31 is coupled to the non-inverting input of firstoperational amplifier 11 for providing a variable reactance (C1, C2, . .. , Cn). A variable impedance frequency control (VIFC) network is alsocoupled to the non-inverting input of first operational amplifier 11. Asecond variable impedance network, variable impedance bandwidth control(VIBC) network, is coupled to the input port of the filter circuit. Theresistance of the second variable impedance network (VIBC) can beadjusted to vary the bandwidth of the filter. The tunable filter circuitfurther includes a second operation amplifier 12, resistors (R3; R4; andR5), static capacitor (C), input port (In) and output port (Out) asshown. The frequency response of the tunable filter for three tuningstates, R₁, R₂, and R_(n) are shown in FIG. 6B.

FIG. 7 illustrates a pair of tunable filters 73 configured in an RFsystem for use in filtering signals in the transmit and receive paths.One tunable filter is connected to the output of a power amplifier 74and a port on a switch 72. The second tunable filter is connected to theinput port of a low noise amplifier 75 and a port on the switch 72. Alook-up table 77 is resident in a processor 76 and contains tuning stateinformation to tune the filters for transmit and receive applications asa function of frequency channels.

FIG. 8 illustrates a total of eight tunable filters configured with fourtunable filters 83(a-d) connected to four power amplifiers 84(a-d) andfour tunable filters 83(e-h) connected to four low noise amplifiers85(a-d). One port of each filter is connected to a single eight throwswitch 81 which is in turn connected to a single antenna 82. A look-uptable 87 is resident in a processor 86 and contains tuning stateinformation to tune the filters for transmit and receive applications asa function of frequency channels.

FIG. 9 illustrates a tunable filter module 90. The pair of op amps 91;92are shown along with three tunable components or circuits (VIBC; firstVIFC and second VIFC as shown). Two tunable components or circuits(first and second VIFCs) are used for frequency control while a thirdtunable component or circuit (VIBC) is used for adjusting the bandwidthof the filter response. A processor 96 is shown along with control lines98 to send control signals from the processor to the tunable componentsor circuits.

FIG. 10A illustrates a method of implementing a look-up table in memoryin a processor to control a PA, tunable filter, tunable attenuator, andswitch. The look-up table is populated with parameters that will providefor uniform output power from the PA/tunable filter combination as thetunable filter is adjusted for frequency. Here, antenna 105 is coupledto switch 104, tunable attenuator 103, tunable filter 102, and poweramplifier 101 in that order as shown. Processor 106 is coupled to eachof the switch, tunable attenuator, tunable filter, and power amplifiervia control lines 107 extending therebetween. The look-up table 108 isloaded in the processor and configured to tune filter response andadjust output. FIG. 10B shows insertion loss as a function of frequencyin accordance with the embodiment of FIG. 10A, including a constantamplitude response as shown.

FIG. 11A illustrates a method of implementing an algorithm 112 in memoryin a processor 106 to control a PA 101, tunable filter 102, tunableattenuator 103, power detector 111 and switch 104. The algorithm sendscontrol signals to the components via control lines 107 to provide foruniform output power from the PA/tunable filter combination as thetunable filter is adjusted for frequency. Antenna 105 is coupled asshown. Here, the algorithm is loaded in the processor and configured totune the filter response and adjust output power. FIG. 11B showsinsertion loss as a function of frequency in accordance with thatillustrated in FIG. 11A, including a constant amplitude response.

FIG. 12A illustrates a method of implementing an algorithm 112 in memoryin a processor 106 to control an LNA 121 and tunable filter 102. Thealgorithm sends control signals via control lines 107 to the components121; 102; 104 to provide for uniform receive power from the LNA/tunablefilter combination as the tunable filter is adjusted for frequency.Antenna 105 is coupled to the circuit as shown. FIG. 12B shows insertionloss as a function of frequency in accordance with that illustrated inFIG. 12A, including a constant amplitude response.

FIG. 13A illustrates the front-end of a communication system wheretunable filters 102 a; 102 b are implemented in both the transmit andreceive chains. A look-up table 108 is implemented in memory resident ina processor 106, with the look-up table containing parameters used tocontrol the Tx and Rx tunable filters along with the PA 101 and LNA 131and tunable attenuator 103 as a function of frequency channelinformation. Also shown is power detector 111, switch 104, and antenna105, which are arranged in similar fashion as other embodimentsdisclosed herein. FIG. 13B shows insertion loss as a function offrequency in accordance with that illustrated in FIG. 13A, includingfilter slope adjustment as a function of frequency channels, includingsteeper slope (SLOPE 1), and relaxed slope (SLOPE 2).

FIG. 14A illustrates the front-end of a communication system wheretunable filters 102 a; 102 b are implemented in both the transmit andreceive chains. A look-up table 108 is implemented in memory resident ina processor 106, with the look-up table containing parameters used tocontrol the Tx and Rx tunable filters along with the PA 101 and LNA 131and tunable attenuator 103 as a function of frequency channelinformation. In this configuration, the filter response in both thetransmit and receive chains are compensated based upon the antennafrequency response. Also shown is power detector 111, switch 104, andantenna 105, which are arranged in similar fashion as other embodimentsdisclosed herein. FIG. 14B shows insertion loss as a function offrequency in accordance with that illustrated in FIG. 14A, includingfilter slope adjustment as a function of frequency channel compensatedfor antenna frequency response, including transmit antenna response(Tx-A), receive antenna response (Rx-A), filter plus antenna response(F+A), and filter response (F) as shown.

FIG. 15 illustrates a tunable filter and antenna circuit simulation. Thereturn loss and efficiency of the antenna. The antenna is an IMD(Isolated Magnetic Dipole) “M” type element which exhibits a narrow bandfrequency response.

FIG. 16 illustrates a tunable filter topology, showing an operationalamplifier and components to provide the tuning function. A switch isshown which provides the capability of utilizing or by-passing acapacitor. Resistor R78 and capacitor C47 are shown, and these twocomponents are used to adjust the frequency response of the filter.

FIG. 17 illustrates a detailed circuit topology of the operationalamplifier. Two follower circuits are highlighted and the circuittopology of the follower circuit is shown.

FIG. 18 illustrates the transmission loss characteristics of the tunablefilter and antenna circuit shown in FIG. 15. Three frequency tuningstates are shown for the tunable filter without the antenna responseincluded, and are compared to a SAW filter optimized for Band 8 (925 to960 MHz). A second plot containing transmission loss characteristics ofthe tunable filter and antenna response is also shown, and are comparedto a SAW filter optimized for Band 8 (925 to 960 MHz). The roll-off intransmission loss of the tunable filter and antenna combination isgreater when compared to the tunable filter only, and provides a bettermatch to the transmission loss characteristics of the SAW filter.

1-12. (canceled)
 13. A tunable filter comprising: a first operationalamplifier comprising a non-inverting input and an output thereof, and asecond operational amplifier comprising a non-inverting input and anoutput thereof, the output of the first operational amplifier connectedto the non-inverting input of the second operational amplifier at anoutput of the tunable filter; the output of the second operationalamplifier connected to the non-inverting input of the first operationalamplifier at an input of the tunable filter; wherein the output of thesecond operational amplifier is connected to a variable resistorcircuit, the variable resistor circuit connected to the non-invertinginput of the first operational amplifier.
 14. The tunable filter ofclaim 13, wherein a change in resistance of the variable resistor isoperable to result in an inverse change in frequency of a filterresponse of the tunable filter.
 15. The tunable filter of claim 13,wherein the variable resistor circuit comprises a plurality of resistorscoupled to a multi-port switch.
 16. The tunable filter of claim 13,further comprising a tunable capacitor coupled in between thenon-inverting input of the first operational amplifier and ground. 17.The tunable filter of claim 16, wherein the variable resistor circuit isconnected in a series configuration with the non-inverting input of thefirst operational amplifier prior to a junction where the tunablecapacitor is connected between the non-inverting input of the firstoperation amplifier and ground.
 18. The tunable filter of claim 13,further comprising a second variable resistor circuit coupled to thenon-inverting input of the first operational amplifier.
 19. Acommunication system comprising the tunable filter of claim
 13. 20. Thecommunication system of claim 19, wherein the communication systemcomprises an antenna.
 21. The communication system of claim 20, whereinthe antenna is an isolated magnetic dipole.
 22. The communication systemof claim 20, wherein the tunable filter is disposed in a receive chain.23. The communication system of claim 20, wherein the tunable filter isdisposed in a transmit chain.